2H2 + O2 = 2H2O + ENERGY
This is an example of an exothermic reaction, a reaction that produces energy. It is also an example of what is called a combustion reaction, where a substance (in this case hydrogen gas) is combined with oxygen. You are probably familiar with this reaction through two tragic examples of the unleashed energy of the combustion reaction of hydrogen, the Hindenburg, and the spaceshuttle Challenger. Hydrogen Fuel?
Yes - hydrogen is a good, clean fuel, producing only water as a by-product. Unfortunately it produces so much energy that it can get out of control, resulting in an explosion. But let's forget about that explosive part for a minute and think about the possibilities - Hydrogen as a New Clean Fuel - it could be the end of the energy crisis - but where would we get the hydrogen?
Can we create Hydrogen from Water?
Oh Yes! It's the same chemical reaction, but run in reverse:
2H2O + ENERGY = 2H2 + O2
Notice now that the requirement is for energy to be ADDED TO the reactants. This is an example of an Endothermic reaction. This means that we could use Water as a Fuel! IF (and this is a big if) we could find an easy way to convert the water to hydrogen and oxygen, then the hydrogen could be used as a clean fuel. One way to convert Water to Hydrogen and Oxygen is through the process of Electrolysis - using electricity as the source of energy to drive the reaction. Let's take a look at what that might look like:


Isn't this rather circular?
Using Energy to break water to form hydrogen to combine oxygen to form Energy - in this way is rather circular. In fact, because of the laws of thermodynamics, you can't break even in this exchange of energy. However, there exist better ways to disassemble water - namely using CATALYSIS. What does a catalyst do?
A catalyst is a chemical compound that acts to speed up a reaction, but in the process is not itself changed. Therefore the catalyst, at the end of the reaction, is free to act again to assist another reactant through the reaction. Catalysts work by lowering the energy barrier between the reactants and the products. In this case:
2H2O + ENERGY = 2H2 + O2
where it normally takes a tremendous amount of energy to convert reactants to products - the addition of a catalyst can decrease the amount of energy required and therefore speed the reaction up!
2H2O + CATALYST+ energy = 2H2 + O2 + CATALYST
Does this catalyst really exist?
Sort of.....